usb: doc: phy-mxs: update binding for adding anatop phandle

Add anatop phandle which is used to access anatop registers to
control PHY's power and other USB operations.
Signed-off-by: Peter Chen <peter.chen@freescale.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>

Documentation/devicetree/bindings/usb/mxs-phy.txt

@@ -8,10 +8,12 @@ Required properties:
   "fsl,imx23-usbphy" is still a fallback for other strings
 - reg: Should contain registers location and length
 - interrupts: Should contain phy interrupt
+- fsl,anatop: phandle for anatop register, it is only for imx6 SoC series
 
 Example:
 usbphy1: usbphy@020c9000 {
 	compatible = "fsl,imx6q-usbphy", "fsl,imx23-usbphy";
 	reg = <0x020c9000 0x1000>;
 	interrupts = <0 44 0x04>;
+	fsl,anatop = <&anatop>;
 };